1. Field of the Invention
The present invention relates to a lithographic printing method involving digital plate-making on a press and a lithographic printing apparatus therefor. More particularly, the present invention relates to a plate-making/printing method which comprises performing plate-making with an oil-based ink, followed by printing, and a printing apparatus therefor, whereby a printing plate having a good image quality can be provided and a print having a good image quality can be provided.
2. Description of the Related Art
In the field of lithographic printing, ink receptive areas and ink repellent areas are formed on a surface of a printing plate in accordance with an original image. Printing ink adheres to the ink receptive areas to effect printing. Ordinarily, hydrophilic areas and oleophilic (ink receptive) areas are formed in image patterns on the surface of a printing plate, and the hydrophilic areas are converted to oil-based ink repellent areas by applying dampening water thereto.
Conventional image formation on a printing plate (plate-making) is carried out by exposing a silver salt photographic film with the desired image in an analog or digital manner, exposing a photopolymer material (original printing plate precursor) containing a diazo resin or a photopolymerizable polymer to the silver halide photographic film, and then removing the photosensitive material by dissolving out the non-image areas. This removal process is carried out mainly by using an alkaline solution.
With recent improvements in digital recording technology and the demand for more efficient printing processes, various methods where digital image information is directly recorded on a plate material have been proposed. These methods include technologies referred to a CTP (computer-to-plate) and a DDPP (digital direct printing plate) method. These methods typically involve an image recording system having a photon mode or heating mode using a laser beam. Some of these methods have been put to practical use.
However, after the image is recorded on a plate using either the photon mode or the heating mode, the non-image areas are dissolved out by treating the plate with an alkaline developer. This method results in the discharge of an alkaline waste liquid, which is environmentally undesirable.
As a means of effecting the printing process at an enhanced efficiency there is proposed a system in which image recording is effected on the press. The foregoing method involving the use of laser may be employed. However, this method requires an expensive and large-sized apparatus. Thus, a system utilizing an ink jet method which employs an inexpensive and compact image recording apparatus has been attempted
JP-A-4-97848 (The term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d) discloses a method which comprises forming a lipophilic or hydrophilic image on a plate drum which is hydrophilic or lipophilic on the surface thereof instead of the conventional plate cylinder by an ink jet process, and then removing the image after printing to clean the plate drum. However, this method is disadvantageous in that the desired removability of printed image (i.e., cleanability) and press life cannot be accomplished at the same time. In order to form a printed image having a prolonged press life on the plate cylinder, it is necessary that an ink containing a resin in a relatively high concentration be used. Thus, the inkjet recording device for forming a printed image uses a resin solution as an ink. Accordingly, the resin can be easily solidified due to the evaporation of solvent at the nozzle, deteriorating the stability in the ejection of ink. As a result, a good image can hardly be obtained.
Further, JP-A-64-27953 discloses a plate-making method which comprises recording an image of a lipophilic wax ink on a hydrophilic printing plate material by an ink jet recording process. However, this method is disadvantageous in that since the image is formed of a wax, the resulting image area has a reduced mechanical strength. This method is also disadvantageous in that the adhesion of the image area to the hydrophilic surface of the printing plate material is insufficient, reducing the press life.
The present invention has been worked out paying attention to the foregoing problems. A first object of the present invention is to provide an on-press recording type lithographic printing method and apparatus for use with a digital recording system requiring no development. A second object of the present invention is to provide an on-press recording type lithographic printing method and apparatus capable of providing a large number of prints having sharp images of high quality in a simple and inexpensive manner.
The foregoing object of the present invention is accomplished by the following aspects of the present invention.
(1) An on-press recording type lithographic printing method involving an ink jet process which comprises ejecting an oil-based ink onto a printing plate material mounted on the plate cylinder of a press utilizing an electrostatic field according to signals of image data, whereby an image is directly formed on the surface of said printing plate precursor, thereby preparing a printing plate which is then used as it is to effect lithographic printing, characterized in that when any abnormality occurs, the formation of image is suspended and/or the cause of abnormality is removed.
(2) The on-press recording type lithographic printing method according to Clause (1), wherein the oil-based ink is a dispersion comprising resin particles which are solid and hydrophobic at least at ordinary temperature dispersed in a nonaqueous solvent having an electrical specific resistance of 109 xcexa9-cm or more and a dielectric constant of 3.5 or less.
(3) An on-press recording type lithographic printing apparatus comprising an ink jet recording device provided with a recording head which ejects an oil-based ink utilizing an electrostatic field as an image forming means for directly forming an image on a printing plate material according to signals of image data utilizing an electrostatic field and a lithographic printing means for effecting lithographic printing using a printing plate formed by the image forming means, characterized in that there are provided an abnormality sensing means and/or a means of removing the cause of abnormality and the image forming means is suspended and/or the means of removing the cause of abnormality is operated at least temporarily according to the output from the abnormality sensing means.
(4) The on-press recording type lithographic printing apparatus according to Clause 3, wherein as the abnormality sensing means there is provided a recording head foreign matter attachment sensing device for sensing the attachment of foreign matters to the ink jet recording device.
(5) The on-press recording type lithographic printing apparatus according to Clause 3, wherein as the abnormality sensing means there is provided a dust sensing device for sensing dust inside the on-press recording type lithographic printing apparatus and/or on the printing plate material.
(6) The on-press recording type lithographic printing apparatus according to Clause 3, wherein as the abnormality sensing means there is provided a vibration sensing device for sensing the vibration of the on-press recording type lithographic printing apparatus and/or the recording head.
(7) The on-press recording type lithographic printing apparatus according to Clauses 3 to 7, wherein the oil-based ink is a dispersion comprising resin particles which are solid and hydrophobic at least at ordinary temperature dispersed in a nonaqueous solvent having an electrical specific resistance of 109 xcexa9-cm or more and a dielectric constant of 3.5 or less.
(8) The on-press recording type lithographic printing apparatus according to Clauses 3 to 7, wherein there is provided a device for fixing the ink.
(9) The on-press recording type lithographic printing apparatus according to Clauses 3 to 8, wherein the image forming means comprises a printing plate material surface dust removing means for removing dust present on the surface of the printing plate material before and/or during the recording of an image on the printing plate material.
(10) The on-press recording type lithographic printing apparatus according to Clauses 3 to 9, wherein the image forming means performs main scanning during the recording of an image onto the printing plate material, by rotation of the plate cylinder in which the printing plate material is mounted.
(11) The on-press recording type lithographic printing apparatus according to Clause 10, wherein the recording head comprises a single channel head or multiple channel head and the recording head moves in the direction parallel to the axis of the plate cylinder during the recording of an image on the printing plate material to effect subsidiary scanning.
(12) The on-press recording type lithographic printing apparatus according to Clause 10, wherein the recording head comprises a full-line head having a length which is almost the same as the width of the plate cylinder.
(13) The on-press recording type lithographic printing apparatus according to Clauses 3 to 12, wherein the ink jet recording device comprises an ink supplying means for supplying an ink into the recording head.
(14) The on-press recording type lithographic printing apparatus according to Clause 13, wherein there is provided an ink recovering means for recovering an ink from the recording head and ink circulation is carried out by the ink supplying means and the ink recovering means.
(15) The on-press recording type lithographic printing apparatus according to Clauses 3 to 14, wherein there is provided an ink stirring means in an ink tank housing an oil-based ink.
(16) The on-press recording type lithographic printing apparatus according to Clauses 3 to 15, wherein there is provided an ink temperature controlling means in the ink tank housing an oil-based ink.
(17) The on-press recording type lithographic printing apparatus according to Clauses 3 to 16, wherein there is provided an ink concentration controlling means for controlling the concentration of the ink.
(18) The on-press recording type lithographic printing apparatus according to Clauses 3 to 17, wherein the ink jet recording device is provided with a recording head moving means for moving the recording head close to the plate cylinder during the recording of an image on the printing plate material and moving the recording head away from the plate cylinder except during the recording of an image on the printing plate material.
(19) The on-press recording type lithographic printing apparatus according to Clauses 3 to 18, wherein the image forming means is provided with a recording head cleaning means for cleaning the recording head at least after the termination of plate making.
(20) The on-press recording type lithographic printing apparatus according to Clause 3, wherein there is provided a means for protecting the recording head.
(21) The on-press recording type lithographic printing apparatus according to Clause 19, wherein the recording head cleaning means applies a voltage to the recording head while the recording head is being dipped in a cleaning fluid.